1. Field of the Invention
The invention relates to a wheel hub device, more particularly to a wheel hub device for a bicycle, which permits smooth idle rotation of a hub shell relative to a driving barrel, and which ensures non-operation of the pedals when the cyclist drags the bicycle backwards so as to prevent possible injury to the cyclist.
2. Description of the Related Art
FIGS. 1 and 2 show a freewheeling hub device disclosed in co-pending U.S. patent application Ser. No. 09/761908 filed by the applicant on Jan. 17, 2001. As shown, the freewheeling hub device includes a hub shell 2 mounted rotatably on an axle 1 of a bicycle (not shown), a driving barrel 3 coupled to a lateral end of the hub shell 2, a limiting element 4 disposed to limit a lateral end of the driving barrel 3, a coupling socket 5 inserted via an opposite lateral end of the hub shell 2 and engaged threadedly with the driving barrel 3, a coil spring 6 having one end connected to an outer wall surface of the coupling socket 5, a connecting spring 7 connected to an inner wall surface of the coupling socket 5, a sliding member 8 connected to the connecting spring 7, a limiting block 9, and a cup 401 disposed to limit a lateral end of the limiting block 9. The hub shell 2 has an annular wall 201 with an inner wall surface 202. A tapered inner friction wall portion 203 and a shoulder portion 204 are disposed on the inner wall surface 202. The driving barrel 3 includes a toothed head portion 301 and an externally threaded portion 302 that extends axially from one end of the head portion 301. The coupling socket 5 has a tapered outer friction wall portion 501 formed on the outer wall surface thereof, and an internally threaded portion 502 threadedly engageable with the externally threaded portion 302. The coil spring 6 has a securing end 601 and an abutting end 602, and spirals clockwise from the securing end 601 to the abutting end 602 with a gradually enlarged curvature radius. The securing end 601 is insertably retained in an insert hole 503 in the coupling socket 5 via an insert pin 601xe2x80x2, whereas the abutting end 602 abuts against the shoulder portion 204. The inner wall surface of the coupling socket 5 is in frictional contact with one end of the connecting spring 7. The sliding member 8 is connected to an opposite end of the connecting spring 7, and is disposed to fit into a non-circular hole in the cup 401 such that the sliding member 8 can displace only along an axial direction.
When the pedals of the bicycle are moved forwardly to rotate the driving barrel 3 corresponding to the direction of advancement of the bicycle, the coupling socket 5 is brought to displace toward the head portion 301 of the driving barrel 3 (the coil spring 6 being in a compressed state at this time), and the outer friction wall portion 501 engages the inner friction wall portion 203 of the hub shell 2. As such, the driving barrel 3, the coupling socket 5 and the hub shell 2 are coupled as a whole to enable the driving barrel 3 and the hub shell 2 to rotate synchronously to thereby move the bicycle forward. Conversely, when the pedals are moved backwards, the outer friction wall portion 501 will disengage from the inner friction wall portion 203 so that the hub shell 2 performs idle rotation relative to the driving barrel 3.
When the bicycle continues to move forward due to the action of inertia resulting from previous forward pedaling, the ideal situation is that the shoulder portion 204 retains the abutting end 602 of the coil spring 6 in a direction corresponding to the direction of advancement of the bicycle (as indicated by the arrow in FIG. 1), and that, when the abutting end 602 is being retained, due to the transmission of torque generated by the coil spring 6, the securing end 601 forces the coupling socket 5 to rotate relative to the connecting spring 7 (the torque of the coil spring 6 must be greater than the frictional force between the coupling socket 5 and the connecting spring 7), and the outer friction wall portion 501 quickly disengages from the inner friction wall portion 203 to allow the hub shell 2 to rotate freely in the idle state. Under the condition that the outer and inner friction wall portions 501, 203 are not engaged, when the cyclist gets off the bicycle to drag the bicycle backwards, the pedals will not turn to strike the cyclist""s legs. However, the hub device described above may suffer from the following drawbacks:
1. As the coil spring 6 does not have a uniform curvature radius, during the transmission of torque, the several coils immediately before the abutting end 602 will also be retained by the shoulder portion 204 so that the abutting end 602 is squeezed outwardly toward the inner wall surface 202 of the hub shell 2 and may come into contact therewith to obstruct smooth rotation of the hub shell 2 and transmission of torque of the coil spring 6.
2. During the inertial forward motion of the bicycle, in order for the outer friction wall portion 501 to disengage from the inner friction wall portion 203 smoothly, the coil spring 6 must produce a torque that can resist the frictional force between the coupling socket 5 and the connecting spring 7. However, the retaining force provided by the shoulder portion 204 against the abutting end 602 is not sufficient to enable the securing end 601 to produce a sufficient torque to force the coupling socket 5 to rotate relative to the connecting spring 7, with the loss of torque during transmission taken into account. Thus, the outer friction wall portion 501 may not disengage from the inner friction wall portion 203 smoothly.
Therefore, the main object of the present invention is to provide a wheel hub device for a bicycle, which permits smooth idle rotation of a hub shell relative to a driving barrel, and which ensures non-operation of the pedals when the cyclist drags the bicycle backwards so as to prevent possible injury to the cyclist.
Accordingly, a wheel hub device according to the present invention is adapted for use in a bicycle which includes an axle extending in an axial direction, a sprocket wheel mounted coaxially around and rotatable relative to the axle, and a bicycle wheel which is mounted on the wheel hub device for rotation. The wheel hub device includes a hub shell, a driving barrel, a coupling socket, a sliding member, an associating spring, and a coil spring. The hub shell is adapted to be rotatably mounted on the axle, and has first and second lateral ends which are disposed opposite to each other in the axial direction, and an inner peripheral wall surface which extends in the axial direction to communicate the first and second lateral ends and which defines a through hole. The inner peripheral wall surface has a first wall portion and a second wall portion respectively proximate to the first and second lateral ends, and an intermediate wall portion which is disposed between the first and second wall portions and which defines a shoulder facing towards the second lateral end. The driving barrel is adapted to be rotatably mounted on the axle and is rotatable relative to the hub shell. The driving barrel includes a head portion which is disposed outwardly of the first lateral end and which is adapted to be rotated with the sprocket wheel, and an externally threaded portion which is disposed opposite to the head portion in the axial direction and which extends into the through hole so as to be surrounded by the intermediate wall portion. The coupling socket is disposed rotatably and movably in the through hole in the axial direction, and is adapted to be rotatably mounted around the axle. The coupling socket includes an engaging end, a coupling end, and an abutment member. The engaging end has a friction wall surface that confronts the intermediate wall portion, and an internally threaded portion that is disposed opposite to the friction wall surface in a direction radial to the axial direction to be movable in the axial direction between a fully engaged position, where the externally threaded portion is in a full threaded engagement with the internally threaded portion as a result of a clockwise rotation of the driving barrel relative to the coupling socket, which corresponds to a forward movement of the bicycle wheel, and where the friction wall surface is frictionally engaged with the intermediate wall portion as a result of the full threaded engagement, and a partially engaged position, where the externally threaded portion is in a partial threaded engagement with the internally threaded portion as a result of a screwing-out movement of the internally threaded portion relative to the externally threaded portion, and where the friction wall surface is disengaged from the intermediate wall portion. The coupling end is disposed opposite to the engaging end in the axial direction away from the driving barrel. The abutment member is disposed on and extends radially and outwardly from the coupling socket, and is located between the engaging end and the coupling end so as to spacedly face and move toward the shoulder in the axial direction when the internally threaded portion is moved to the fully engaged position. The sliding member is adapted to be sleeved on the axle, and is movable in the axial direction. The associating spring is mounted in the through hole and is adapted to be disposed around the axle. The associating spring is interposed between the coupling socket and the sliding member such that, during the screwing-out movement, the associating spring cooperates with the sliding member to stabilize movement of the coupling socket in the axial direction. The coil spring is adapted to be disposed around the axle, and is interposed between the shoulder and the abutment member. The coil spring has a first biasing force, and includes a depressed end disposed to bias against movement of the abutment member towards the shoulder and against a second biasing force of the associating spring, and an abutting end disposed opposite to the depressed end in the axial direction to abut against the shoulder such that when the clockwise rotation of the driving barrel is terminated suddenly while the forward movement of the bicycle continues as a result of inertia, the internally threaded portion rotates relative to the externally threaded portion in a screwing-out movement so as to be disposed in the partially engaged position, and a frictional force is simultaneously imparted to the abutting end to enable the depressed end to drag the abutment member and the coupling end of the coupling socket to rotate relative to the associating spring while the depressed end is biasing against movement of the abutment member and against the second biasing force so as to expedite the screwing-out movement.